Wafers and masks, including those non-silicon wafers and photomasks used in, e.g., hard disk drive read/write heads and optical device fabrication, typically are inspected for defects and/or measured for device critical dimension control using electron beam (e-beam) or ion beam systems utilizing beam scanning for image-based defect analysis or measurements. The systems used include scanning electron microscopes (SEM) for failure analysis and defect review, focused ion beam (FIB) systems, and critical dimension scanning electron microscopes (CDSEM). During the inspection or measurement process of a feature of interest on a specimen placed on a mechanical stage, the system acquires an image of a portion of the targeted area on the specimen after the stage is moved to the predetermined coordinates, and the system software compares the image to a pre-stored image template for location registration verification so that the correct feature of interest can be identified prior to the final inspection or measurement execution. The acquired image after stage move is compared with the stored image template and the unique pattern recognition feature in the image is identified using pattern recognition principles. If the comparison fails, the system understands that the first stage move is not successfully bringing the electron/ion imaging optics close to the targeted area. The test might subsequently start executing a so-called stage search movement, usually a spiral search method with gradual radial distance increase accompanied by stage move around the first stage move coordinates, to determine whether any subsequent image acquired after each stage search contains the unique pattern in the image templates. Stage search and the accompanying image acquisition can be time-consuming, adversely affecting specimen inspection or measurement throughput and thus the production cycle time.
As understood herein, complicating the above process is the fact that during the imaging acquisition process it is possible for certain types of specimen, e.g., photoresist with the combination of either top or bottom antireflective coating layers (TARC or BARC) or photoresist on insulator under layer on a wafer, or chrome patterned layer on glass substrate for a photomask, to become charged as the result of being repeatedly exposed to the imaging beam, e.g., during the accompanying image acquisition after each stage move in the stage search procedure. Charging of the specimen is frequently encountered when inspection/failure analysis SEM, CDSEM and FIB inspection fixtures are used. The charge on the specimen changes the contrast of the image vis-á-vis the first image obtained from a fresh uncharged specimen. This charge-induced contrast change in turn results in false coordination verification failures, precipitating unnecessary stage searches and results in pattern recognition failures during automatic specimen inspection and measurement. Various pattern recognition algorithms have been developed that attempt to filter out contrast changes caused by charging but these algorithms do not work well, they generally are case-specific, and they typically require considerable user experience to function.